The present invention relates to integrated circuits for controlling the charging and discharging of a rechargeable battery.
Many portable electronic systems are powered by rechargeable batteries, typically Ni-Cd batteries. Such batteries have the advantage that they are portable, non-contaminating, relatively weight-efficient, and can be charged and discharged many times. However, Ni-Cd batteries also have some significant quirks which make optimal control difficult.
First, there is the well-known "memory" effect. If a Ni-Cd battery is repeatedly only partially discharged before recharging, the microstructure of the battery will gradually adapt, so that the battery's full capacity is no longer available.
Another non-linear effect is that total amount of energy which can be withdrawn in a discharge cycle is somewhat dependent on the rate of discharge.
A further non-linear effect is that, if the battery is completely discharged, e.g. into a dead short circuit, the microstructure of the battery will change to reduce the total capacity.
A further non-linear effect is the use of "trickle charge" currents. A battery which is already fully charged can be maintained at maximum readiness by applying a very small current to the battery..sup.1 This phenomenon is very well known in lead acid batteries and also applies to Ni-Cd batteries. FNT .sup.1 This current would typically be of the order of "0.1 C," i.e. an amount of current equal to the rated battery capacity divided by 10 hours.
Another perverse characteristic of Ni-Cd batteries is that the voltage of a Ni-Cd will drop at full charge. Thus, in alternative embodiments, the chip of the present invention can be configured to watch for this voltage fall-off as a charging cycle comes to an end.
These difficulties with managing rechargeable batteries have long been generally known. For large rechargeable battery installations, expensive controllers (typically costing $5,000 or more, in 1989 dollars) have been proposed by others. Such controllers attempt to monitor the discharge characteristics of a bank of batteries and control the charging current and/or charging time to maximize the available battery capacity.
In addition, the battery characteristics will also be affected greatly by temperature. For example, a rate of discharge which is not excessive at one temperature may be excessive at another temperature. All of these effects are somewhat difficult to model theoretically, but can be fitted to an empirical model with reasonable accuracy.
These battery management issues apply not only to high performance batteries, such as Ni-Cd or other high-performance battery types, but also to lead acid batteries. Lead acid batteries have a much lower cost-per-unit battery capacity (amp-hours at rated voltage) than do Ni-Cd batteries, but lead acid batteries provide a much lower amount of battery capacity per unit weight and also a much lower amount of battery capacity per unit volume. However, many lead acid battery installations are used in contexts where weight and volume are essentially unlimited. In such cases, the designer of a lead acid battery installation can provide some additional margin for error by increasing the size of the battery banks used.
The present invention provides very sophisticated battery monitoring functions in a single integrated circuit. Thus, reliable portable electronic modules, powered by rechargeable batteries, can be configured with greatly improved battery management capabilities.
This is particularly valuable in electronic devices where sudden battery failure could cause an intolerable loss of data. One key example of this type is lap-top computers. Another important class of applications is hand-held scientific or medical instruments. Another important class of applications is in hand-held portable data collection terminals. Another important class of applications is in military and police equipment, such as portable radio transceivers. A very important class of applications is in hand-held portable tools for commercial and industrial use.
The present invention provides a battery management chip which, in the presently preferred embodiment, incorporates several novel features. Not all of the features described are necessary to the claimed invention, but the combination of all of the features described is particularly advantageous.
The presently preferred embodiment of the battery manager chip includes an on-chip PMOS pull-up transistor, which can turn charging current to the battery on or off. (A corresponding logic output is also provided to control discrete switching transistors if desired for a larger current capability.)
One of the innovative features of the present invention is the provision of an integrated circuit with a comparator having two inputs for differential temperature-sensing base on inputs from two different sensors. Thus, one thermocouple or thermistor can be placed in close thermal contact with the casing of the battery, while the other thermistor is exposed to ambient temperature. This permits a temperature rise in the battery to be sensed. This is very useful in controlling charging characteristics. Otherwise, the rate of charging current may be excessive under a low ambient temperature and lower than necessary under high ambient temperature.
Another of the innovative features of at least one embodiment disclosed herein is that, in a portable module, the battery manager integrated circuit controls the charging and discharging of the rechargeable battery which powers the whole module, and is also connected to draw power from an external power supply, and is also connected to draw very small amounts of current from a third, stable battery, preferably a lithium battery, which is not necessarily rechargeable.
The integrated circuit of the presently preferred embodiment has also been designed to provide versatility for other analog interface or control functions in addition to battery management. For example, the two analog sensor interface circuits can be used not only for differential temperature assessing of battery temperature rise, but can also be used for inlet and outlet coolant temperature sensing and air-cooled or even liquid-cooled systems. Note that these inputs can also be used for pressure sensors, fluid level detectors, fluid flow detectors, or other sensor input interfaces.
In the presently preferred embodiment, these inputs are connected to thermistor temperature sensors. A thermistor is a temperature-dependent variable resistor, which therefore requires a biased current input to provide a voltage output. The biased current would normally be provided by an off-chip source. Alternatively, for some applications it may be preferable to provide temperature sensing from a thermal couple plus an instrumentation amplifier.
One of the key novel teachings is a battery manager integrated circuit which is configured for interface to a microprocessor. This provides system configurations to be implemented, wherein a system microprocessor can intelligently monitor battery-charged state, among other characteristics.
In particular, one characteristic of the integrated circuit of the presently preferred embodiment which gives additional versatility is its capability for automatic and manual modes. That is, the integrated circuit of the presently preferred embodiment can be configured so that it acts independently to disconnect current sourced from the battery when the battery becomes excessively low; or it can be configured to act simply as a microprocessor peripheral, so that the battery manager chip provides warnings but does not implement connection or disconnection actions.
Another significant teaching of the presently preferred embodiment is a battery manager integrated circuit which includes an on-chip bandgap voltage reference. The circuitry for bandgap voltage references is conventional, and a variety of circuit configurations are very well-known, but bandgap voltage references normally have a significant power consumption. However, the precise voltage reference derived from such a circuit is extremely useful in performing the battery control function as described below. (In alternative embodiments, the on-chip bandgap voltage reference can be replaced with expedients such as an off-chip zener diode voltage reference.)
A further novel teaching set forth in the present application is an integrated circuit which includes a crystal-controlled oscillator for precise time measurement. Crystal-controlled oscillators are normally fairly power-hungry circuits, and such circuits would not normally be used in the low-power part unless needed. However, according to this innovative teaching, the precise time integration provided by the crystal oscillator is significantly advantageous, since it permits accurate time integration to derive the present state of the battery after multiple charge and discharge cycles. In addition, in the presently preferred embodiment, a low-power crystal-controlled oscillator is used.
In the presently preferred embodiment, the battery manager integrated circuit is configured as an n-well part. The advantage of this is that the substrate is held at ground. This is advantageous in handling the multiple power input described below. The open-drain outputs used can be pulled high without abnormal problems. In particular, if the charging current supply voltage goes above the on-chip VDD voltage, as may well occur, this chip configuration will avoid any problem of junctions thereby being forward biased. Thus, the oscillator configuration of the presently preferred embodiment is the dual circuit to that shown in the DSC-74 application cited more specifically in the preferred embodiments.
A further novel teaching disclosed herein is the integrated circuit battery manager which can predict an imminent low-battery condition without waiting until voltage measurements show that the battery is actually dying. The precise measurement capabilities of the presently preferred embodiment permit this to be achieved. In addition, the presently preferred embodiment of the battery manager chip includes two low-battery outputs. These are referred to in the text below, as the "low-battery" and "MIN-battery" status bits. These may be thought of as warning conditions and alarm conditions respectively.
A further innovative feature of the battery manager chip of the presently preferred embodiment is the capability for both on-chip and off-chip switching of both charge and discharge currents. Whenever sizable currents need to be handled, it will of course be preferable to use a discrete transistor, such as a discrete power PMOS device controlled by logic signals generated from the battery manager chip. However, in addition, in low-current applications (e.g., where the current switched is of the order of hundreds of milliamps or less), the currents needed may be within the capability of on-chip to these PMOS drivers. Another consideration is whether the voltage drop incurred by going on-chip drivers, and then off-chip again, is acceptable. The present invention provides capability for both configurations, and therefore provides additional flexibility to the end user.
Another notable feature of the presently preferred embodiment is the multiplicity of comparators provided. In the presently preferred embodiment, four comparators are provided, two single-ended and two differential. The single-ended comparators, in the presently preferred embodiment, are used for the tests which generate the max-battery and min-battery signals. The differential comparators are used for temperature sensing, according to the innovative teachings set forth above, and for detection of a low-battery condition. Note that a differential comparator is not strictly necessary for detection in the low-battery condition. However, the provision of the additional differential comparator provides additional versatility for this chip to be used in applications beyond those limited to battery management, as described above.
A further notable feature of the presently preferred embodiment is that the comparators all have a one-way trip operation. That is, electrically, these comparators are combined with other circuit elements to achieve significant hysteresis. For example, when the operating conditions are just on the margin of tripping the low-voltage detection, it would be undesirable to have the corresponding signal turning on and off intermittently. Thus, in the presently preferred embodiment, circuit hysteresis in included (e.g. by including a latch in the circuit), so that the user normally has to service the interrupt to clear the trouble signal.
In an alternative embodiment, instead of using two differential comparators as shown, one of the pins is used instead for a programmable interrupt. The programmable interrupt, in addition to the primary interrupt, can be used to program the chip for sensing a particular condition. Thus, for example, the output on a programmable interrupt can be used to drive an LED or an audible alarm, to flag some particular anticipated condition for user response.